An aluminum flake pigment is used in wide-ranging fields as a metallic pigment contained in a paint composition, an ink composition or a resin composition.
In general, the aluminum flake pigment is obtained by wet-milling aluminum powder serving as a raw material together with a grinding lubricant and an organic solvent using a grinder or pulverizer such as a ball mill or an attriter having grinding media thereby flaking the aluminum powder.
The metallic effect of a paint film mainly depends on the shape, the surface smoothness, the mean particle size, the particle size distribution, the average thickness, the thickness distribution, the aspect ratio etc. of the aluminum flake pigment, which are adjusted by a combination of the characteristics of the raw aluminum powder and grinding conditions.
While the metallic effect is visually recognized as the combination of brightness, lightness, sparkling effect and the like, there has generally been a strong tendency among demanders to desire a paint film having high brightness. In general, the brightness of the paint film and the mean particle size of the aluminum flake pigment are so correlative with each other that the brightness is increased as the mean particle size is increased.
If the aluminum flake pigment has a large mean particle size, however, the orientation of the aluminum flake pigment tends to be irregular in formation of the paint film, the aluminum flake pigment may protrude from the paint film to give seeding on the surface of the paint film, and the paint film may excessively glitter to result in an unpreferable design.
Therefore, development of an aluminum flake pigment having a small mean particle size as well as high brightness is desired, and development efforts are made in various scenes.
For example, Japanese Patent Laying-Open No. 8-170034 discloses that it is possible to provide a paint film simultaneously with strong brightness and an excellent appearance by a metallic pigment composition containing (A) 100 parts by solid weight of film-forming resin and (B) 0.1 to 30 parts by weight of an aluminum flake pigment having a mean particle size (D50) in the range of 20±5 μm, an average particle thickness (t) in the range of 0.5 to 1 μm and a gradient (n) of at least 2.7 in a Rosin-Rammler diagram.
Further, Japanese Patent Laying-Open No. 11-152423 discloses an aluminum flake pigment exhibiting high brightness and excellent circulation resistance, having an average aluminum flake thickness (t) in the range of 0.2 to 0.7 μm, a mean particle size (D50) in the range of 4 to 20 μm, an aspect ratio (D50/t) in the range of 15 to 50 and an uniformity index (n) of at least 2.4.
Also when the aluminum flake pigment disclosed in either gazette is employed, however, the requirement for an aluminum flake pigment having a small mean particle size as well as high brightness is not yet sufficiently satisfied.